add some docs about USB serial devices..

serial.md

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+Overview of USB serial
+======================
+
+There are lots of different features each brand.  Finding out those features are difficult so hopefully this will bring the information together in a single spot.
+
+## Major Players
+
+1. FTDI (uftdi)
+2. Prolific (uplcom)
+
+## Minor Players
+
+1. WinChipHead (CH340) (uchcom)
+2. Silicon Laboratories (uslcom)
+
+There are many additional third tier players as well.
+
+Details
+-------
+
+# FTDI
+
+They are probably most common.
+
+## Pros
+
+Wide availability.
+
+## Cons
+
+They have played dirt w/ their offical drivers to combat conterfit devices from bricking devices, to sending junk on the serial port when detecting couterfit devices.
+
+# Prolific
+
+## Pros
+
+## Cons
+
+Special hardware needed to program serial number.  And Windows only software.
+
+# Silicon Laboratories
+
+## Pros
+
+Can program serial number: https://github.com/emaste/cp210x-cfg
+## Cons